The invention concerns a method for controlling downshifting steps in automatic or automated transmissions, so as to make it possible to improve the driving pleasure and the security of the vehicle.
In most methods for managing gear shifting in automatic mode, for automatic transmissions as well as for automated transmissions, gear shifting is decided by a computer as a function of the speed of the vehicle and the load of the engine. Most often, these criteria are translated in the form of gear shifting laws, which are represented in the form of curves constructed in a plane having the speed of the vehicle at the wheels Vveh in abscissa and the degree of depression of the acceleration pedal in ordinate.
The general principle of the gear shifting laws is proposed in FIG. 1, in which curves C1 and C2 represent gear shifting laws by downshifting from N to N−1 and from N+1 to N, respectively.
According to this principle, it appears that a single shifting of gears by downshifting is possible at one time. Indeed, to pass from a transmission ratio N+1 to a transmission ratio N−1, the vehicle must undergo a certain deceleration so that the computer decides the passage from the transmission ratio N+1 to the transmission ratio N, then the passage from the transmission ratio N to the transmission ratio N−1. For example, in the case where the driver decides to slow down the vehicle by releasing the acceleration pedal, then by braking, it is observed that we start at a point A to reach a point B which corresponds to the fact that the driver has released the acceleration pedal, then, when the speed of the vehicle at the wheels Vveh cuts the curve C2 at point C, the computer decides the passage from the transmission ratio N+1 to the transmission ratio N and, finally, when the speed of the vehicle at the wheels Vveh cuts the curve C1 at point D, the computer decides the passage from the transmission ratio N to the transmission ratio N−1. Overall, a few seconds go by to pass from the transmission ratio N+1 to the transmission ratio N−1.
It is observed also that downshifting, according to the general principle of the gear shifting laws, does not take into account parameters other than the speed of the vehicle and the load of the vehicle, such as the intensity of braking, the duration of braking, or the driving style of the driver.
In summary, the general principle of the gear shifting laws such as described above does not make it possible to anticipate downshifting operations that generate engine braking, in the case where the driver wishes to slow down the vehicle or obtain some acceleration.
In order to remedy this drawback, recent systems for controlling automatic gearboxes, based on the calculation in real time of the engine speed under which a downshifting is decided, have been developed. It is the case, for example, with the patent EP 0991880 B1 which claims a method for controlling downshifting making it possible, among others, to anticipate downshifting as a function of the load of the vehicle, the duration of braking the intensity of braking, the driving style of the driver, and the speed of the vehicle. The method for controlling downshift described in the patent EP 0991880 B1 consists in determining, during braking, as a function of the intensity and the duration of braking, of the declivity of the road, and of the driving style of the vehicle, the threshold value of the speed of the input shaft of the gearbox Ωthreshold, under which the classic law for shifting gears is no longer used, and under which the anticipated downshifting from a transmission ratio N to a transmission ratio N−1 is imposed.